In a diseased heart the natural electrical system of the heart is disrupted so that the heart cannot sustain a normal pumping rhythm by itself. A pacemaker is therefore required to provide timed electrical stimulation signals to either or both of the atrium and ventricle of the heart. It is well-known that pacemakers may also sense natural electrical activity in the atrium or ventricle and automatically inhibit stimulation or pacing in response to appropriately timed natural electrical activity.
In operation, a pacemaker is typically implanted within the body in the chest cavity of the patient and an electrically conducting pacing lead is positioned within the body between the pacemaker and the heart, for example by passing the electrode along a vein to an interior region of the heart at which sensing and pacing will occur. Typical pacemaking leads are constructed with one or more coils of conducting wire which are covered by an outer sheath of an insulating material, for example polyurethane, to form the body of the lead. The lead has a proximal end with a connector which is plugged into the pacemaker and a distal electrode end which is affixed to heart tissue to stimulate the tissue and detect electrical cardiac signals.
The distal end of typical pacemaking leads includes an electrically conducting tip which is held against the cardiac tissue. If the lead is unipolar, this tip electrode defines the active part of a circuit between the heart and the pacemaker and the electrically conducting housing of the pacemaker provides the ground for this circuit. The unipolar lead thus has a single coil of wire which connects the pacemaker at one end to the pacing and sensing tip electrode at the other.
A bipolar pacing lead has the above-described tip electrode disposed at its distal end and an associated ring electrode disposed on the body of the lead, usually approximately 2 centimeters from the tip. The bipolar lead body typically includes two coaxially disposed conducting coils which are insulated from one another. One of the coils is conductively connected to the tip electrode and the other of the coils is conductively connected to the ring electrode. Pacing and sensing of cardiac tissue is thus achieved between the active tip electrode and passive ring electrode.
Bipolar pacing leads with the above-described structure have been found adequate for sensing electrical activity of the ventricle and pacing the ventricle. In operation, the pacing lead is disposed so that the tip electrode contacts the wall of the heart in the ventricle and senses electrical activity in a field defined between the tip and ring electrodes. This electrode structure has been found adequate to detect electrical QRS signals (R-waves) which cause natural contraction of the ventricle. If the tip electrode of the pacing lead is placed against the right ventricle, such QRS signals have a sufficiently large amplitude to be relatively easily detected by circuitry in the remote implanted pacemaker. Although natural electrical activity occurs synchronously in the atrium and ventricle, it has been found that the natural atrial signal or P-wave is so low in energy that it does not interfere with the detection of the relatively powerful QRS signal in the ventricle. Thus, when sensing in the ventricle, the amplifiers of the signal detection circuitry of the pacemaker may relatively easily detect QRS signals.
However, when the pacing lead is disposed in the atrium in order to detect atrial P-waves, it has been found that the high energy QRS signal is also detected and thus interferes with the detection of the atrial signal. Relatively complex circuitry has been utilized in the art in an effort to distinguish P-waves and QRS signals in the atrium, so that the pacemaker will respond only to the detection of P-waves. Such circuitry adds to the expense and complexity of the pacemaker and increases the battery power requirements, thus reducing the useful life of the pacemaker within the body. The added complexity of such circuitry is also undesirable in that it reduces the reliability of the pacing system. Moreover, in discriminating between R-waves and P-waves, there is always the chance that an error will be made and the patient's heart will be unnecessarily stimulated.
Accordingly, it is an object of the invention to provide a pacing lead with a structure that will allow P-wave signals to be detected in the atrium with enhanced sensitivity and that will discriminate such atrial signals from R-waves of the ventricle.
Another object of the invention is to provide such a pacing lead with a tip portion which supports two closely spaced electrodes that contact cardiac tissue, for example tissue in the atrium, and detect P-wave signals in the near field but do not detect QRS signals in the far field of the lead.
A further object of the invention is to provide such a bipolar pacing lead which includes a tip electrode at its distal end for stimulating the atrium and an associated coaxially disposed inner electrode in insulated relation with the tip electrode for sensing electrical signals in conjunction with the tip electrode.
Another object of the invention is to provide such an electrode with bifilar coils independently insulated and disposed in intermeshing relationship within the body of the pacing lead and respectively conductively connected to the two electrodes at the distal end of the lead.
A further object of the invention is to provide a pacing lead with enhanced sensitivity which includes a tip electrode disposed in insulated relation to an associated affixation element having electrically conducting tines, the tines and the tip electrode being electrically connected to the pacer to provide the pacing and sensing functions.
These and other objects of the invention will be understood by reference to the drawings and to the detailed description of preferred embodiments, wherein like elements are identified by the same reference numerals.